This invention relates generally to fans that move air. More specifically, the invention relates to an improved blade for a centrifugal flow fan having forward curved blades.
A centrifugal flow fan, sometimes called a "squirrel cage" fan, has a generally cylindrical outer envelope with a plurality of blades arranged around and generally parallel to the fan axis of rotation. Air flows axially into the fan and discharges radially through the blades. A centrifugal fan frequently is contained within a scroll housing that serves to direct the air discharged from the fan into a single direction. A forward curved centrifugal flow fan has blades that curve in the direction of rotation of the fan from the leading edge to the trailing edge of the blade.
The general principles of fluid flow, including laminar and turbulent boundary layer flow and boundary layer separation, are well known in the art. As related to fans, premature boundary layer separation on the blade surfaces results in increased radiated noise levels and reduced air moving efficiency due to increased aerodynamic drag. Unlike an aircraft airfoil, which is generally subject to premature boundary layer separation only on its suction (or upper) surface, a forward curved centrifugal fan blade can suffer premature boundary layer separation on both its suction and pressure surfaces. This premature separation is due to a number of factors including the great amount of flow turning the air experiences as it enters the fan blades and mismatch of blade incidence angles.
In the fans commonly used in heating ventilation and air conditioning (HVAC) applications, the blade tip speeds are relatively low, in the range below Mach 0.15. In such a fan, the air flow is principally laminar in nature and thus is more susceptible to premature boundary layer separation than if the flow were turbulent. Premature boundary layer separation increases both the blade drag and radiated noise level of a fan.